1. Field of the Invention
This invention relates generally to superconductive materials and more specifically to a method and apparatus for making elongated, flexible superconductive strips and wires.
2. Description of the Prior Art
Superconductive materials are materials that have the characteristic of conducting an electric current virtually without resistance. In such superconductive materials, once a flow of electric current has been started, it will continue flowing in a closed loop indefinitely, even after removal of the electric source.
The superconductive materials known at present include metals, for example, iridium, lead, mercury, niobium, tin, tantalum, and vanadium, and many alloys, metal oxides, and other chemical compounds, in which their resistivities practically disappear at temperatures approaching absolute zero. The temperature at which such a material undergoes a transition from a normal conductor to a superconductor is called the transition temperature or critical temperature, T.sub.c. A desirable goal of researchers in this field is, of course, to develop superconductive materials that have higher critical temperatures, preferably in the range of normal room temperatures. That goal has, so far, been elusive, and it might not be reached. However, it is significant that materials having critical temperatures near 100K have been formulated, an achievement that at the present time is considered to be very significant and exciting.
Unfortunately, the superconductive materials that have been formulated to date with the higher transition or critical temperatures, such as, Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8, are very similar to ceramic oxides and are very brittle. Therefore, they have little or no capability of bending or being wound into a coil, such as for a solenoid, which is one of the most desirable apparatus formats for using superconductivity in practical applications. Japanese researchers at the Technological University of Nahaoga reported in the Apr. 14, 1988, issue of NATURE, Vol. 332, p. 575, that they had succeeded in making glass panes or sheets of this material by melting various combinations of bismuth, calcium, strontium, aluminum, lead, and copper oxides at 1150.degree. C., quenching the melts by pouring them on an iron plate, quickly pressing them into a slab, and subsequently annealing them. However, the goal of forming these materials into elongated, superconductive wires or strips has also remained elusive prior to this invention.
There have been several attempts to fabricate elongated strips or wires of superconductive materials. For example, U.S. Pat. No. 3,796,553, issued to J. Daunt, and U.S. Pat. No. 3,815,224, issued to M. Pickus et al., disclose processes of preparing a porous matrix of ductile and thermally conductive metal, infiltrating the voids in the matrix with a ductile, superconductive material, and then drawing the material into long, narrow wires. However, these methods are apparently limited to ductile superconductive materials; but, as mentioned above, is not a characteristic of the brittle, high-T.sub.c superconducting oxide materials.
U.S. Pat. No. 4,336,280, issued to A. Moller, discloses a process of depositing Nb and Ge layers on a long substrate from a gas mixture of Nb, Ge, H.sub.2, and water vapor. U.S. Pat. No. 4,596,207, issued to A. Witt et al., while related to semiconductors rather than superconductors, is of general interest in showing a melt-spinning technique for forming a composite, elongated metal ribbon with one or more layers of different metals or alloys deposited in laminar fashion on a spinning platform.
The May 1988 issue of Stanford Observer reported the development of a melt drawing method of fabricating up to 15 in. of bismuth, calcium, strontium, copper, and oxygen (Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8) wire. A laser is used in that process to melt the surface of a source rod that is cut from a conventional pellet of the material so that a seed crystal can be introduced into the melted end and drawn out at a controlled rate to produce the crystalline wire strand.
M. Mimura, et al., in their article "Improvement of the Critical Current Density in the Silver Sheathed Bi-Pb-Sr-Ca-Cu-O superconducting Tape," Appl. Phys. Lett., Vol. 54, No. 16, pp. 1582-84, Apr. 17, 1989, describe a process of packing powders of the superconducting material in silver tubes or sheaths, then cold working them with a swaging machine and a grooved rolling machine into short (3 cm) wire shapes. These short wire lengths were then subjected to a combination process of alternate heat treatments and cold working treatments, ultimately forming them into tapes or strips. This process, M. Mimura, et al. report, improves the coupling between grains of the superconducting material, which contributes to better critical current density (J.sub.c) at the critical or transition temperature (T.sub.c). Unfortunately, that process is hot conducive to continuous production of continuous strands of indeterminate lengths.
S. Jin et al., in their article "Fabrication of Dense Ba.sub.2 YCu.sub.3 O.sub.7-.delta. Superconductor Wire by Molten Oxide Processing," Appl. Phys. Lett., Vol. 51, No. 12, Sep. 21, 1987, described their investigations of three different processes, including melt drawing, melt spinning, and preform-wire melting. The melt drawing process involved heating a bar-shaped, pressed compact of fine Ba.sub.2 YCu.sub.3 O.sub.7-.delta. powder to about 1200.degree.-1300.degree. C. with a blow torch and then pulling to draw the material into wire forms about 10 mm long. The melt spinning technique was similar to the Witt et al. U.S. Pat. No. 4,596,207 described above to the extent that a droplet of molten material (Ba.sub.2 YCu.sub.3 O.sub.7-.delta. ) is deposited on the surface of a rapidly rotating mandrel to obtain a wire strand 40 mm long. The preform-wire melting process described by Jin et al. is perhaps the most relevant to the present invention in that it uses a metal wire core initially as a substrate for the metal oxide material. The metal oxide (Ba.sub.2 YCu.sub.3 O.sub.7-.delta. ) powder was mixed with a binder, coated onto the surface of the metal wire core, dried, and sintered into a hard shell coating around the metal wire core. Finally, the outside surface of the metal oxide shell was melted by rapidly moving a torch flame over the shell surface. A 25-mm strand was fabricated, although Jin et al. suggest that longer, perhaps even continuous, wires or ribbons might be obtainable with this process.
All the attempts described above to obtain long, bendable or flexible wire strands of superconductive material have moved the technology forward, but they still have their respective problems. For example, the respective melt drawing processes described above are limited to lengths that can be pulled in single crystals, which are too short and fragile to be of much practical use. The melt spinning processes are fragile, limited in length, and not suitable for mass production and commercial use. The Pickus et al. and Daunt processes require ductile superconductive materials, so they are of little value for the higher-T.sub.c oxide materials, which are hard and rigid or brittle. The gas deposition technique of the Moller patent described above is limited to certain materials that can be deposited from gaseous forms. Conventional vapor depositions, sputtering, and the like are somewhat inefficient, expensive, and wasteful of both source material and energy.